For a number of years penetrants, such as dye penetrants and fluorescent penetrants, have been used for finding cracks and other flaws in the surfaces of workpieces. Generally, a penetrant material is infiltrated into the cracks of a workpiece and after the bulk of the material is wiped away, the residual material which is remaining in the crack is sought out optically. Various means, such as dyes, powders and fluorescing mediums are used to enhance the visibility of these penetrants. There are, of course, a considerable number of currently used commercially available penetrants and over the years even many more have been experimented with or offered to the public for sale. And even within the context of a supposedly constant and known penetrant, there may be variations in the extent to which the penetrant will reveal cracks. Such variations can result when the penetrant composition inadvertently changes, or the method of application changes over time.
In many applications it is of the most vital importance that the cracks beyond a certain size be found. Therefore, there has been a continuing need for a standard test method for evaluating a penetrant, and in particular to evaluating the sensitivity of a penetrant in finding particular cracks. Substantial effort in this direction is indicated by the 1965 U.S. Military Specification MIL-I-8963 (ASG) which was never officially issued. The specificiation discloses test panels made by a combination of nickel and chromium plating of brass panels. The panels are bent in curved dies to crack them at the plated surface. The crack type (size) varies according to the plating bath composition and operating parameters which are used.
The military specification reflects an ongoing endeavor to provide a standard test specimen with a standard crack pattern. Others have made specimens by mechanically or thermally fatigue cracking a substrate piece of material. However, the cracks in such specimens are generally too large and tend to be too variable from one specimen to the next. Further, there has always been a problem in removing the penetrant from the bottom of certain types of cracks, especially the foregoing, after a penetrant has been assessed. If residue from the penetrant accumulates in the crack, the crack is thereby made shallower and this change in character alters reproducibility in use of the test specimen.
A significant advance in making test specimen is revealed in Japanese patent publication No. 14633/1974 "Comparative Test. Specimen for Penetrant Test", by S. Shimizu, one of the applicants herein. Described in the Shimizu invention, a test specimen consists of a flat brass plate with one surface having a very precise thickness nickel plating and very thin chromium overcoat. Stress is imposed from the rear to crack the plating and to form a multiplicity of cracks running perpendicular to the length which is bent. The crack sizes correspond with the thickness of the nickel plated layer which varied controllably in different specimens from 2-50 micrometers, at least.
The foregoing panels have worked well but it is necessary to successively use panels with different plating thicknesses to determine the sensitivity of a particular penetrant. It has been recognized that improvements could be achieved if a range of crack sizes could be controllably provided on a single panel. Specifically, an improved panel will have a plating which varies gradually in thickness along its length. Such a panel is called herein a "tapered test panel" and is disclosed in Japanese patent applications No. 32145/1980 and No. 23161/1980, both of Y. Natori. The first patent discloses simply a tapered test panel without indicating the manner in which it might be plated or cracked. One can assume that it would be made in the same manner as described in the Shimizu patent. (According to Shimizu, plates are cracked by bending them in any arbitrary way, such as by pressing the plate over a rod laid transverse to its length.) The second patent of Natori discloses how a tapered thickness plating is detained on a test panel: The substrate is placed in a plating bath and there is a movable screen between the substrate and the cathode, which screen is raised or lowered across the front of the substrate to alter the current distribution and thereby the plating thickness.
While tapered test panels are thus revealed, in fact, they have not been able to be cracked easily in a manner which produces uniformly spaced cracks. That is, in the practice of Shimizu or the Military Specification with the uniform thickness platings, the bending moment which was applied to the panel, such as by simply stressing it over a pivot, was not so critical. No matter where the sufficient tensile stress is created in the nickel plating at the surface, any cracks would have depths which correspond with the thickness of the plating, which thickness is constant across the panel. But, in a tapered panel cracks must be uniformly present in one panel to the next. The reason is that when such panels are used, the distance from a reference end to the point where the crack penetrant visibility demarcates is measured. The demarcation point is gaged subjectively or objectively according to the intensity of fluorescence, color, etc. It is obvious that for accurate measurement there must be a certain minimum pitch of cracks. But less obviously, the quantity of cracking at any location influences the perception of the demarcation point. As an illustration, if at a given local region along the length of a first panel there is but single crack, but in a second similar panel there are a multiplicity of closely spaced cracks, then the intensity of signal will be greater in the second panel. If the second panel is mistakenly judged to indicate the penetrant finds a certain size crack whereas the first does not, such panels will have failed in their intended purpose of being standards.
In fact, all the conventional means of bending have been tried on tapered test panels but there has been variation in the placement and density of cracks. Such variation can be attributed both to bending techniques of insufficient precision and to the tapered panel being a somewhat complex varying shape structure. Because cracking has been non-uniform, tapered test panels have not been acceptable as standards.